Recreation
Management
United States Department of Agriculture
Forest Service
TechTips
DEP
AR
UR
E
EST SERVICE
FOR
TMENT OF AGRICULT
Technology &
Development Program
March 2008
2300
0823 1301—SDTDC
Magnesium Chloride Toilet Vault Treatment:
A Means To Inhibit Freezing in Outdoor Toilet Vaults
Brenda Land, Sanitary Engineer
Joe Fleming, Mechanical Engineering Technician
INTRODUCTION
Toilet vaults in colder climates are prone to
freezing in the winter, which is often the period
of their heaviest use. Throughout the Forest
Service, U.S. Department of Agriculture, and other
agencies, sweet smelling toilets (SSTs) are widely
used. These toilets have a lined containment
vault, which stores 500 to 1,000 gallons of waste.
The SST has a large vent stack that draws a
draft through the toilet riser and up to the outside
where the odor is less noticeable. While the toilet’s
design goal is to reduce odor, the draft through
the vault draws in outside air and that accelerates
freezing of the vault contents. Freezing of the
waste can form a stalagmite that actually blocks
the toilet riser, rendering the toilet unusable and
unsanitary. Once the toilet riser is blocked, the only
practical way to make it usable again is to have it
steam cleaned and pumped. The cost of cleaning
and pumping each toilet vault is typically more
than $500, and it may need to be repeated several
times each season.
PURPOSE AND SCOPE
The San Dimas Technology & Development
Center (SDTDC) was asked to address the frozen
waste issue. The project’s goal was to develop a
system or technique that will prevent—or at least
inhibit—stalagmite formation in the vaults. If the
contents can be kept in a fluid condition, the waste
will spread evenly throughout the vault and not
form a frozen cone. Naturally, any method selected
For additional information, contact: Recreation Management Program Leader, San Dimas Technology & Development Center,
444 East Bonita Avenue, San Dimas, CA 91773-3198; Phone 909-599-1267; TDD; 909-599-2357; FAX: 909-592-2309
Lotus Notes: Mailroom WO SDTDC@FSNOTES • Intranet (web site): http://fsweb.sdtdc.wo.fs.fed.us • Internet e-mail: mailroom_wo_sdtdc@fs.fed.us
1
needs to be economically feasible and competitive
with the current practice of steam cleaning and
pumping.
Concentration Rock Salt(NaCl) Magnesium
(lb/gallon)
Freezing Point Chloride (MgCl2)
Degrees F [C] Freezing Point
DISCUSSION
Small, portable outhouses and recreational-vehicle
commodes often use a liquid antifreeze chemical
to prevent freezing and reduce odor, but the cost
of these products when applied to a vault toilet
is too high to compete with the current practice.
Volatile compounds like methanol could be added
periodically to keep the waste fluid but storage,
handling, and safety concerns rule them out. We
(SDTDC staff) considered propane- and oil-fueled
heaters and concluded that safety concerns were
too risky, and the cost of fuel to maintain the
vault temperature above freezing was too high
to compete with cleaning and pumping. We also
considered submersible fish pond and livestock
watering trough heaters. They may be appropriate
for some locations, but not where there is minimal
solar radiation and no grid power available. We
considered small windmills, but the price is high
and some locations do not get enough wind during
the winter to make adequate power. We could
not find any self-starting portable generator sized
appropriately for a single or dual vault, although
if four or more vaults were very close to each
other, it could be economical to power several
submersible heaters with a small generator. We
would still need to enclose the generator and
provide venting for exhaust and adequate fuel
for long-term operation. With all economic and
safety issues considered, chemical treatment with
common road salts looked like the most practical
way to start testing. Magnesium chloride (MgCl2)
offers the lowest freezing point of the salts that
are generally used by State and municipal road
maintenance agencies. MgCl2 dissolves readily in
water, and it is generally accepted by municipal
and local wastewater treatment facilities. However,
before using it, consult the local authority and the
product Material Safety Data Sheet.
0.0
32 [0]
32 [0]
0.5
26 [-3]
26 [-3]
1.0
19 [-7]
18 [-8]
1.5
11 [-12]
0 [-18]
2.0
0 [-18]
-20 [-29]
Figure 1—Freezing points.
TEST LOCATION
The Chugach National Forest in Region 10
(Alaska) has many recreational sites that get heavy
use during the winter season. The maritime climate
that prevails through much of the forest makes
it ideal for winter recreational activities, since it
receives abundant snowfall yet the temperatures
are more moderate than the interior areas of the
State. Turnagain Pass recreational area is very
popular with both snowmobile and nonmotorized
sports enthusiasts. The area is divided by a major
road, and the motorized activities are limited to one
side. Each side has two duplex vault toilets, and
there is no grid power available.
Figure 1 compares the freezing points of MgCl2
and sodium chloride (NaCl) in solution with water.
INITIAL TESTS
During the 2004-2005 winter season, we tried a
MgCl2 brine solution in two of the vaults, but—
due to a design problem on the vault cleanout
hatch—the vaults filled with water and froze
before more salt could be added. Consequently,
these toilets were closed for the rest of the winter.
Temperature dataloggers were placed in two of
the vaults that did not have flooding problems.
One datalogger was in a control (unmodified) vault
and the other was placed in a vault that had the
vent stack capped off. We wanted to measure the
temperature difference caused by the draft, and
were able to get some data during the 2004-2005
winter. A nearby weather station also provided
ambient temperature data, and the results are
shown in figure 2.
Note: The eutectic point for NaCl is 2.00 pounds
per gallon and for MgCl2 it is 2.25 pounds per
gallon. The freezing point will rise with the addition
of more salt beyond these points.
Figure 2 shows the vented vault temperature
changes (vault 1) were more volatile and, in some
cases, they dropped as much as 15 degrees
Fahrenheit below the unvented vault (vault 2).
2
60
Vault 2
Vault 1
weather station
50
Temperature ( deg F)
40
30
20
10
06-Feb-05
30-Jan-05
23-Jan-05
16-Jan-05
09-Jan-05
02-Jan-05
26-Dec-04
-10
19-Dec-04
0
Figure 2—Temperature logs.
However, capping the vent brought odor into the
stall, and the author personally thought it was
unbearable for timespans greater than his ability to
hold his breath.
The vault treated with MgCl2 remained fluid all
season, and it had enough capacity that it did not
need to be pumped. The adjacent control vault
did have freezing problems, and waste built up
below the riser to the point where service was
necessary. Only 200 pounds (four bags) of MgCl2
were required to maintain one vault all winter. The
results from this test only gave us one reference
datum, but in this case, MgCl2 appeared to offer
a viable, economical solution to the problem and
warranted further study.
2005-2006 TEST
In two of the vaults, the cleanouts were modified
with a riser to prevent the flooding that occurred
the prior year, and testing resumed during the
2005-2006 winter season. Due to the odor problem
associated with blocking the vent, we did not
block the vents on either vault. Also, we decided
to try adding MgCl2 directly to the waste instead
of adding it as brine. Generally, there is some
residual liquid in the vault even after it is pumped,
so the MgCl2 was simply dumped in through the
toilet riser as needed to maintain a fluid state.
Only one vault was treated with MgCl2 and the
adjacent vault served as the control.
2006-2007 TEST
We expanded our experiment to include seven of
the eight vaults at Turnagain Pass. We put data
sheets in the closets of each duplex stall and
stocked them with MgCl2 bags. Our intent was
to record the amount of MgCl2 added, and the
intervals. We did not add any liquid to the vaults,
3
and there was very little residual liquid in them
when we added the first dose of one bag per stall
in the first week of November 2006. Subsequently,
two bags were added to each stall the following
week and another bag was added during the last
week of November, for a total of four bags per
stall. Since the vaults were almost completely
empty, there was not enough liquid to dissolve the
MgCl2 and adding more chemical did not help. The
maintenance crew did not want to compromise
capacity by adding liquid, since they felt that would
force them to pump the vaults before the season
was over.
CONCLUSIONS
MgCl2 can be effective at preventing toilet vault
contents from freezing. However, at the start of
winter season precharge the vault with about
10 inches of water below the riser and add 1
to 2 pounds of MgCl2 per gallon, depending on
winter temperatures. Depending on average
temperatures and increasing liquid level in
the vault, use figure 1 to determine how much
additional MgCl2 to add during the winter season.
SDTDC staff thanks Kathie Snodgrass, project
leader, Missoula Technology and Development
Center, and Carl Skustad, recreation operations,
Chugach National Forest, for their review of this
publication.
Magnesium chloride needs a fluid medium to
be effective at thawing solid waste. You should
use about 2 pounds of MgCl2 per gallon of water
to keep the freezing point down around -20
degrees Fahrenheit (-29 degrees Celsius), but
concentrations greater than 2.25 pounds per
gallon will start to increase the freezing point.
Since the salt did not seem to be very helpful when
the content of the vault was already frozen, the
maintenance crew did not add any more MgCl2
after November 26, 2006.
SDTDC’s national publications are available on the
Internet at http://www.fs.fed.us/eng/pubs/
Forest Service and U.S. Department of the Interior,
Bureau of Land Management employees also
can view videos, CDs, and SDTDC’s individual
project pages on their internal computer network at
http://fsweb.sdtdc.wo.fs.fed.us/
ECONOMICS
A full pallet of MgCl2 (49 bags) costs about $2,200
delivered to Anchorage, AK. A pallet can be
purchased from the manufacturer in Salt Lake City,
UT, for about $780 plus shipping. Shipment via
contract carrier on a Government bill of lading to
destinations within the contiguous United States
should cost less. Including shipping to Anchorage,
AK, the MgCl2 treatment cost approximately $182
per vault for an entire season. Compared to $500
per vault for steaming and pumping, we believe
that the salt treatment will be less costly.
For additional information on magnesium chloride
toilet vault treatment, contact Brenda Land or
Joe Fleming at SDTDC. Phone: 909–599–1267.
E-mail: bland@fs.fed.us or jdfleming@fs.fed.us
The information contained in this publication has been developed for the guidance of employees of the Forest Service, U.S. Department of Agriculture, its contractors, and cooperating
Federal and State agencies. The Forest Service assumes no responsibility for the interpretation or use of this information by other than its own employees. The use of trade, firm, or
corporation names is for the information and convenience of the reader. Such use does not
constitute an official evaluation, conclusion, recommendation, endorsement, or approval of
any product or service to the exclusion of others that may be suitable.
activities on the basis of race, color, national origin, age, disability, and where applicable,
sex, marital status, familial status, parental status, religion, sexual orientation, genetic information, political beliefs, reprisal, or because all or part of an individual’s income is derived from
any public assistance program. (Not all prohibited bases apply to all programs.) Persons with
disabilities who require alternative means for communication of program information (Braille,
large print, audiotape, etc.) should contact USDA’s TARGET Center at (202) 720-2600 (voice
and TDD). To file a complaint of discrimination, write USDA, Director, Office of Civil Rights,
1400 Independence Avenue, S.W., Washington, D.C. 20250-9410, or call (800) 795-3272
(voice) or (202) 720-6382 (TDD). USDA is an equal opportunity provider and employer.
The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and
4